🐞 Node.js – Debugger – Step-by-Step Debugging in Node.js Applications

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🧲 Introduction – Why Debugging Matters in Node.js?

Debugging is a critical part of software development. In Node.js, bugs often emerge from asynchronous logic, unhandled errors, or logic flaws in server-side code. The Node.js Debugger helps developers identify and fix such issues by pausing execution, inspecting variables, and stepping through code—similar to browser dev tools, but for your server code.

🎯 In this guide, you’ll learn:

• How to start the built-in Node.js debugger
• Use breakpoints, step through code, and inspect variables
• Debug using Chrome DevTools and VS Code
• Best practices to simplify debugging

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⚙️ Starting the Node.js Debugger from Terminal

Node.js includes a built-in CLI debugger. To launch it, use:

node inspect app.js

📌 inspect enables debugging mode and pauses at the first line of app.js.

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🐞 Example – Simple Buggy Script (app.js)

function multiply(a, b) {
  return a * b;
}

function calculateTotal(price, qty) {
  let total = multiply(price, qty);
  console.log("Total:", total);
}

calculateTotal(5, "3");

This will print:

Total: 15

💥 But "3" is a string. Let’s debug this.

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🛠️ Step-by-Step Debugging with CLI

Launch debugger:

node inspect app.js

Then use these commands:

Command	Description
cont	Continue execution until next breakpoint
next	Step to the next line
step	Step into function calls
out	Step out of current function
watch('x')	Watch a variable
repl	Enter interactive mode
break 4	Set a breakpoint at line 4

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🧪 Debug with Chrome DevTools

You can also attach Node.js to Chrome’s DevTools using:

node --inspect app.js

🌐 Output:

Debugger listening on ws://127.0.0.1:9229

1. Open chrome://inspect in Chrome.
2. Click “Open dedicated DevTools for Node”.
3. Set breakpoints, inspect scopes, and watch variables.

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💻 Debugging in VS Code (Recommended)

1. Open your Node.js project in VS Code.
2. Go to the Run and Debug tab (left sidebar).
3. Click “create a launch.json file” → Select Node.js.
4. Set breakpoints in the editor by clicking on the gutter.
5. Hit F5 or click Start Debugging.

🎯 VS Code gives:

• Live variable inspection
• Breakpoints
• Call stacks
• Console input/output

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🔐 Useful Debugging Flags in Node.js

Flag	Description
--inspect	Starts debugger protocol on port 9229
--inspect-brk	Breaks before user code starts
--trace-warnings	Show stack trace for process warnings
--trace-deprecation	Logs when deprecated APIs are used

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🧱 Best Practices for Debugging Node.js

✅ Practice	💡 Why It’s Important
Use breakpoints for precision	Avoids unnecessary logs
Replace console.log with debugger	Cleaner and more targeted inspection
Inspect async logic carefully	Common source of Node.js bugs
Use repl for runtime exploration	Test logic and variable values dynamically
Integrate VS Code Debugger	Best UI for debugging complex Node apps

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📌 Summary – Recap & Next Steps

Node.js offers multiple ways to debug—from the CLI to Chrome DevTools and full IDE support in VS Code. Knowing how to use breakpoints, step through async code, and inspect variables makes finding bugs faster and easier.

🔍 Key Takeaways:

• Use node inspect or node --inspect for built-in debugging
• Chrome DevTools offers GUI-based inspection
• VS Code is the best all-in-one debugging tool for Node.js
• Combine breakpoints with repl and watch() for efficient diagnosis

⚙️ Real-world relevance:
Used daily in development for tracing bugs in APIs, middleware, CLI tools, and microservices.

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❓FAQs – Node.js Debugging

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❓ What’s the difference between inspect and inspect-brk?
✅ inspect starts in debug mode; inspect-brk pauses on the first line of code.

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❓ Can I debug async/await functions?
✅ Yes. Modern debuggers like VS Code and Chrome DevTools support stepping through async calls.

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❓ How do I evaluate variables in the debugger?
✅ Use the repl mode or built-in watch panels to type variable names and expressions.

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❓ Can I debug remote Node.js apps?
✅ Yes. Run with --inspect=0.0.0.0:9229 and use a secure SSH tunnel or remote debugger tool.

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❓ Is logging better than debugging?
❌ No. Logs are useful for tracing, but debugger tools let you pause and inspect code at runtime.

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